This invention relates to improved aluminum base alloys having improved hot crack resistance when solidified into cast products.
It is well known that pure aluminum is soft. Thus, in order to produce high strength castings from aluminum, significant amounts of other elements must be added. These chemical additions strengthen the metal considerably, but have the problem that they tend to form low melting point eutectics. The practical consequence of this, from the foundryman""s point of view, is that high strength casting alloys have a wide freezing range.
Relatively pure aluminum alloys (greater than about 99 wt. % Al) freeze over a temperature interval of 5-10xc2x0 C., or less. High strength casting alloys, on the other hand, usually contain less than 95 wt. % Al and freeze over a temperature interval of 50 to 100xc2x0, or more.
During solidification of high strength casting alloys, there is a xe2x80x98mushyxe2x80x99 mixture of solid and liquid metal present in the mold as it cools through this wide freezing range. There is thermal contraction of solid during this cooling and solidification process, and the shrinkage of the solid has the problem that it often results in the formation of hot cracks (hot cracks are also called hot tears). Hot cracking of high strength casting alloys is a serious problem, and has prevented significant commercial use of many alloys, in spite of their excellent properties.
There are few examples of grain refining practices proposed specifically for casting alloys in the prior art. Sigworth and Guzowski (U.S. Pat. Nos. 5,055,256 and 5,180,447; and related foreign patents) discovered that an alloy containing a boride of xe2x80x9cmixedxe2x80x9d composition; (Al,Ti)B2, gave the best results. They proposed a master alloy having a nominal composition of 2.5 wt. % Ti and 2.5% B for best grain refinement in casting alloys. This method of grain refinement did not produce smaller grain sizes, however. It only produced equivalent grain sizes at reduced cost. As such, this method of refinement does not represent a solution to the hot cracking problem in high strength casting alloys.
Arnberg, Halvorsen and Tondel (EP 0553533) have proposed a Sixe2x80x94B alloy refiner for use in casting alloys. Setzer et al (U.S. Pat. No. 5,230,754) have proposed an Alxe2x80x94Srxe2x80x94B master alloy, to simultaneously grain refine and to modify the eutectic in Alxe2x80x94Si alloys. However, these methods do not produce the desired smaller grain sizes.
D. Apelian and J-J. A. Cheng have proposed an Alxe2x80x94Tixe2x80x94Si master alloy (U.S. Pat. No. 4,902,475), but this alloy does not appear to be suitable for grain refinement of high strength casting alloys.
In addition to the patents mentioned above, U.S. Pat. Nos. 3,634,075; 3,676,111; 3,785,807; 3,857,705; 3,933,476; 4,298,408; 4,612,073; 4,748,001; 4,812,290; and 6,073,677 disclose different master alloy compositions and methods of manufacture and use.
Other nucleating particles may be used and include several commercial master alloys for grain refining based on the Alxe2x80x94Tixe2x80x94C system. These master alloys introduce microscopic TiC particles as nucleating agents into the melt. The TiC particles are disclosed in U.S. Pat. Nos. 4,710,348; 4,748,001; 4,873,054; and 5,100,488. Nucleating particles, such as sulfides, phosphides or nitrides (e.g., U.S. Pat. No. 5,100,488) may also be used.
It will be seen that there is still a great need for an improved aluminum alloy and method of grain refinement of high strength, aluminum-based casting alloys which permits use of high strength alloys without the attendant problem of hot cracking.
It is an object of this invention to provide an improved high strength aluminum alloy substantially free from hot cracking.
It is another object of this invention to produce a smaller grain size in cast parts made from high strength, aluminum-based casting alloys.
Yet, it is another object of this invention to reduce or eliminate the problem with hot cracking associated with solidification of these same casting alloys.
Still, it is another object of this invention to produce high strength casting alloys having a better distribution of gas porosity, smaller diameter gas pores, a lessor amount of porosity, and higher fatigue strength.
And still, it is another object of this invention to produce improved grain refinement of high strength, aluminum-based casting alloys at reduced cost.
These and other objects will become apparent from a reading of the specifications, examples and claims appended hereto.
In accordance with these objects there is provided a method of casting an aluminum base alloy to provide a cast product having improved hot crack resistance in the as-cast condition, the method comprising providing a melt of an aluminum base alloy comprised of 4 to less than 5 wt. % Cu, max. 0.1 wt. % Mn, 0.15 to 0.55 wt. % Mg, max. 0.4 wt. % Si, max. 0.2 wt. % Zn, up to 0.4 wt. % Fe, the balance comprised of aluminum, incidental elements and impurities. The dissolved Ti in the melt is maintained in the range of about 0.005 to 0.05 wt. % to improve the resistance of the alloy to hot cracking. A nucleating agent selected from the group consisting of metal carbides, aluminides and borides is added to the melt to provide an undissolved nucleating agent therein in the range of about 0.002 to 0.1 wt. % for grain refining; and the melt is solidified to provide a cast product having a grain size of less than 125 microns, the cast product being free of hot cracks.
Another alloy in accordance with this invention is comprised of 4 to less than 5.2 wt. % Cu, 0.15 to 0.6 wt. % Mn, 0.15 wt. % to 0.6 wt. % Mg, max. 0.15 wt. % Si, max. 0.2 wt. % Zn, up to 0.2 wt. % Fe, 0.4 to 1 wt. % Ag, dissolved Ti in the range of about 0.005 to 0.10 wt. %, and an undissolved nucleating agent in the range of about 0.002 to 0.1 wt. % for grain refining, the balance comprised of aluminum, incidental elements and impurities.
A third alloy in accordance with this invention is comprised of 3.8 to less than 4.6 wt. % Cu, 0.25 to 0.5 wt. % Mn, 0.25 to 0.55 wt. % Mg, max. 0.1 wt. % Si, up to 0.15 wt. % Fe, and 2.5 to 3.5 wt. % Zn, dissolved Ti in the range of about 0.005 to 0.05 wt. %, and an undissolved nucleating agent in the range of about 0.002 to 0.1 wt. % for grain refining, the balance comprised of aluminum, incidental elements and impurities.
Yet another alloy in accordance with this invention is comprised of 4.2 to less than 5 wt. % Cu, 0.2 to 0.5 wt. % Mn, 0.15 to 0.55 wt. % Mg, max. 0.15 wt. % Si, up to 0.2 wt. % Fe, and max. 0.2 wt. % Zn, dissolved Ti in the range of about 0.005 to 0.1 wt. %, and an undissolved nucleating agent in the range of about 0.002 to 0.1 wt. % for grain refining, the balance comprised of aluminum, incidental elements and impurities.
Other alloys in accordance with this invention are comprised as follows:
(1) 4.5 to less than 6.5 wt. % Zn, 0.2 to 0.8 wt. % Mg, max. 0.8% Fe, max. 0.4 wt. % Mn, max. 0.3 wt. % Si, max. 0.5% Cu, and 0.15 to 0.6 wt. % Cr, dissolved Ti in the range of about 0.005 to 0.05 wt. %, and an undissolved nucleating agent in the range of about 0.002 to 0.1 wt. % for grain refining, the balance comprised of aluminum, incidental elements and impurities;
(2) 6 to less than 7.5 wt. % Zn, 0.6 to 1 wt. % Mg, max. 0.15% Fe, max. 0.1 wt. % Mn, max. 0.1 wt. % Cu, max. 0.15 wt. % Si, and 0.06 to 0.4 wt. % Cr, dissolved Ti in the range of about 0.005 to 0.05 wt. %, and an undissolved nucleating agent in the range of about 0.002 to 0.1 wt. % for grain refining, the balance comprised of aluminum, incidental elements and impurities;
(3) 2.7 to less than 4.5 wt. % Zn, 1.4 to less than 2.4 wt. % Mg, max. 1.7% Fe, max. 0.6 wt. % Mn, max. 0.3 wt. % Si, max. 0.4 wt. % Cu, optionally 0.2 to 0.4 wt. % Cr, dissolved Ti in the range of about 0.005 to 0.05 wt. %, and an undissolved nucleating agent in the range of about 0.002 to 0.1 wt. % for grain refining, the balance comprised of aluminum, incidental elements and impurities;
(4) 2.7 to less than 4.5 wt. % Zn, 1.4 to less than 2.4 wt. % Mg, max. 0.8% Fe, 0.2 to less than 0.6 wt. % Mn, max. 0.2 wt. % Si, max. 0.2 wt. % Cu, optionally 0.2 to 0.4 wt. % Cr, dissolved Ti in the range of about 0.005 to 0.05 wt. %, and an undissolved nucleating agent in the range of about 0.002 to 0.1 wt. % for grain refining, the balance comprised of aluminum, incidental elements and impurities;
(5) 4.5 to less than 7 wt. % Zn, 0.25 to less than 0.8 wt. % Mg, max. 1.4% Fe, max. 0.5 wt. % Mn, max. 0.3 wt. % Si, and 0.2 to less than 0.65 wt. % Cu, dissolved Ti in the range of about 0.005 to 0.1 wt. %, and an undissolved nucleating agent in the range of about 0.002 to 0.1 wt. % for grain refining, the balance comprised of aluminum, incidental elements and impurities.
In a preferred embodiment of this invention, the undissolved nucleating agent added to the above alloys is TiB2 or TiC, and the insoluble Ti added is in the range of about 0.003 wt. % to 0.06 wt. %.